Like stated, fresh air coming into the house. Ph is controlled by CO2
exchange. During the winter we close up our homes and run a big heater to keep warm. More CO2
is then in the house which then lowers the pH in our tanks. When warmer weather comes, we open up the windows and doors and with fresh air comes more 'normal' pH levels.
With that said, pH also fluctuates through the day. If this is something you believe you need to track, which it is something I don't believe needs to be, you need to make sure you check it at the same time daily so you are getting a more consistent reading in terms of what is going on in your tank.